Improvements in resolution and sensitivity for magnetometer technologies are beneficial for a variety of fields. Currently, the most sensitive sensors are superconducting quantum interference devices (SQUIDs) and optically pumped magnetometers (OPMs), both with sensitivities below 1 fT/√{square root over (Hz)} when the sensor size is on the order of a centimeter. Specially configured SQUIDs and Hall probes have micrometer scale resolution.
However, as shown in graph 100 of FIG. 1, current magnetometer technologies, including Hall probes, SQUIDs, and vapor cells, all current magnetic field sensors involve a tradeoff between resolution and sensitivity. For instance, the micrometer scale resolution in specially configured SQUIDs and Hall probes is achieved with a sensitivity on the order of nT/√{square root over (Hz)}. This lower sensitivity is a serious limitation for recording magnetic fields of one or a small number of neurons for magnetoencephalography (MEG), for instance, which is a diagnostic technique for studying the brain through measurements of its magnetic field. Single neurons have dimensions on the order of 10-100 μm and are thought to produce (current dipole) ˜10 pT magnetic fields at 100 μm standoff (see simulated neuron image 200 of FIG. 2), which is out of reach of current technologies. Hence, MEG can today only measure averages over large numbers of neurons.
There are two competing technologies for studying brain electrical activity at high resolution: (1) microelectrodes inserted directly into exposed tissue; and (2) MEG with micro-SQUID magnetometers. While microelectrodes are the gold standard for recording electrical activities of neurons, they have several significant problems, including invasiveness, disturbance of neurons, sparseness in space, and the immune system reaction. While there have been initial attempts at imaging in small animal MEG using low-critical temperature (low-Tc) micro-SQUIDs, micro-SQUID performance is not sufficient for micro-MEG (see FIG. 1). Low-Tc is the critical temperature at which the material becomes superconductive, usually requiring liquid helium for cooling. It is also difficult to isolate the cryogenic SQUID from the body temperature biological object.
Nitrogen-vacancy (NV)-diamond magnetometers, which can detect a single electron spin, are promising, but they require irradiation by microwave power and that the target be placed in very close proximity to the NV centers, which may be difficult to achieve. Giant magnetoresistance and similar solid-state devices are also capable of high resolution, but the sensitivity is not sufficient for single neuron detection. A magnetic tunnel junction (MTJ) achieved sensitivity of 133 nT/√{square root over (Hz)} at 1 Hz, but this required flux concentrators of a size of about 1 mm, limiting the resolution to the same order. Due to the limitations of these techniques, magnetic field measurements of brain activity, i.e., conventional MEG, are performed with cm-scale sensors which measure the average field over 104 to 105 neurons. As a result, many important problems which require microscopic resolution remain unsolved. High-resolution, ultra-sensitive magnetometry, capable of detecting a single or a small number of neurons, would greatly aid in improving understanding of brain function and investigating the origins of the MEG signal at a small scale.
One way to improve resolution while maintaining high sensitivity is by miniaturizing OPMs operating in the spin-exchange relaxation-free (SERF) regime. Unfortunately, the sensitivity-resolution tradeoff is far from optimal. When the OPM cell dimension is below 1 mm, spin relaxation is dominated by the spin-destructive collisions on the cell walls and T2˜a2, where T2 is the transverse relaxation time and a is the cell dimension. The sensitivity is then determined both by spin-fluctuation noise, which goes as 1/√{square root over (nVT2)}˜a−5/2, and by photon-shot noise, which goes as
            1              nlT        2              ∼          a              -        3              ,where n is the density, V is the active volume, and l is the path length. Such miniaturization issues are exemplified by the 70 fT/√{square root over (Hz)} sensitivity obtained with a micro-fabricated mm-size OPM, a sensitivity about 100 times worse than that of cm-size OPMs, 0.5 fT/√{square root over (Hz)}, in qualitative agreement with the reported scaling. In addition, the finite thickness of the cell and heat insulating material, not to mention optical design constraints, can substantially increase the stand-off distance for smaller cells, making the miniaturization approach even less effective.
Accordingly, an improved approach to micro-imaging that does not require a significant tradeoff between resolution and sensitivity may be beneficial.